MDS: lock out snapshots until after a flag has been set in the MDSMap

MDS: lock out snapshots until after a flag has been set in the MDSMap

This way users can't put snapshots on their clusters unless they explicitly
ask for them and have seen a warning message. We take a bit of the MDSMap
flags in order to do so. The only thing a little weird here is that anybody
who upgrades to this patch who already has snapshots will hit the EPERM
and have to go through the warning, but it doesn't impact existing snapshots
at all so they should be good.

To go along with this, we add "ever_allowed_snaps" and "explicitly_allowed_snaps"
members to the MDSMap, which are default to false and are set to true
when allow_new_snaps is set. Old maps decoded with new code default to true
and false, respectively, so we can tell.

Fixes: #6332
Signed-off-by: Greg Farnum <greg@inktank.com>
Signed-off-by: Loic Dachary <loic@dachary.org>